Date: 2011jun25
OS: Windows
Q. How does windows check if I have a good internet connection?
A. Its NCSI (Network Connectivity Status Indicator) checks that:
dns.msftncsi.com resolves to 131.107.255.255 using DNS
http://www.msftncsi.com/ncsi.txt contains exactly: Microsoft NCSI
with no newlines or other non-printing characters.
More info
http://technet.microsoft.com/en-us/library/cc766017%28WS.10%29.aspx
Apple devices use
http://captive.apple.com
Which returns "Success" in some HTML.
Another old faithful is
http://example.com